The invention relates to sensors used for dynamically measuring the gauge or runout of an article.
Sensors used to measure the gauge of calendered products, or the runout on elastomeric articles, for example tires, sometimes give erroneous readings because of the dust and dirt encountered in the factory environment in which they are used. If a contact sensor is used, dust can interfere at the contact point between the sensor and article being measured, or with the moving parts of the apparatus itself, and cause drag or stiction. Dust can also interfere with the readings obtained from non-contact optical sensors by interfering with the optical read of the sensor, and some such sensors, for example lasers, respond to markings on the tire, which gives extraneous readings.
As used herein "stiction" refers to momentary resistance to movement, which can be caused by inertia, friction, momentary locking of moving parts, or by other phenomena recognized by those skilled in the art.
Linear variable differential transformer (LVDT) sensors use a ball bearing contact with the object which is measured. Dust sometimes causes stiction in such sensors, and stiction may prevent good resolution in the measurements obtained.
Rotary transformer, stylus type sensors are sometimes used to measure radial runout of tires on force variation machines. As the description indicates, such sensors are shaped like a pencil, are dragged across the surface of the object being measured, and changes in position are recorded by measuring phase angle changes which are converted to a voltage signal.
Ultrasonic sensors provide limited measurements because the spot size for such sensors is very large, and small isolated changes in dimension are not easily detected since small wavelengths must be used for good resolution, and accuracy of many transformers are reduced at small wavelengths. Similarly, capacitive sensors are useful only for large spot size measurements since the capacitive probe must be very small to get good resolution in one sense, but the range of a small capacitive probe is small, and such small probes easily fall out of range.
It is an object of the invention to provide a sensor that can operate in a factory environment and provide a high frequency response, and accurate, reproducible measurements. Other objects of the invention will be apparent from the following description and claims.